Using the Herschel Space Observatory ’ s Heterodyne Instrument for the Far-Infrared ( HIFI ) , we have observed para-chloronium ( H _ { 2 } Cl ^ { + } ) toward six sources in the Galaxy .
We detected interstellar chloronium absorption in foreground molecular clouds along the sight-lines to the bright submillimeter continuum sources Sgr A ( +50 km/s cloud ) and W31C .
Both the para-H _ { 2 } ^ { 35 } Cl ^ { + } and para-H _ { 2 } ^ { 37 } Cl ^ { + } isotopologues were detected , through observations of their 1 _ { 11 } -0 _ { 00 } transitions at rest frequencies of 485.42 and 484.23 GHz , respectively .
For an assumed ortho-to-para ratio of 3 , the observed optical depths imply that chloronium accounts for \sim 4 - 12 \% of chlorine nuclei in the gas phase .
We detected interstellar chloronium emission from two sources in the Orion Molecular Cloud 1 : the Orion Bar photodissociation region and the Orion South condensation .
For an assumed ortho-to-para ratio of 3 for chloronium , the observed emission line fluxes imply total beam-averaged column densities of \sim 2 \times 10 ^ { 13 } cm ^ { -2 } and \sim 1.2 \times 10 ^ { 13 } cm ^ { -2 } , respectively , for chloronium in these two sources .
We obtained upper limits on the para-H _ { 2 } ^ { 35 } Cl ^ { + } line strengths toward H _ { 2 } Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL 2591 .
The chloronium abundances inferred in this study are typically at least a factor \sim 10 larger than the predictions of steady-state theoretical models for the chemistry of interstellar molecules containing chlorine .
Several explanations for this discrepancy were investigated , but none has proven satisfactory , and thus the large observed abundances of chloronium remain puzzling .